There are two ways to connect a pair of speakers together. You can have series or
parallel. Easy enough?

What happens when you hang that twin speaker extension cab off your 1 x
12"?? Hmm... gets a bit more complicated. I've put together an Excel 5
spreadsheet which caters for many different wiring combinations of 2, 3, and 4 speakers.

Given the power output of the amp, it will work out the total impedance that the amp
"sees", and will work out the individual power within each speaker. It has
pictures of the different configurations within the spreadsheet, so you can use these as a
reference for wiring the speakers up too....